1. Field of the Invention
Aspects of the present invention relate to a fuel cell system and an operating method of the same, and more particularly, to a fuel cell system in which catalysts of electrodes are not poisoned and the start-up time is remarkably low, and an operating method of the fuel cell system.
2. Description of the Related Art
A fuel cell is a type of energy generating system in which energy from a chemical reaction between hydrogen contained in a hydrocarbon-based material such as methanol, ethanol or natural gas and oxygen is directly converted into electrical energy.
A fuel cell system includes a fuel cell stack and a fuel processor (FP) as main elements, and further includes a fuel tank, a fuel pump, etc., as sub-elements. The stack forms a main body of the fuel cell and includes a plurality of layers of unit cells which include membrane electrode assemblies (MEA) and separators.
The fuel pump supplies fuel stored in the fuel tank into the FP. The FP reforms and purifies the fuel to generate hydrogen, and supplies the generated hydrogen to the fuel cell stack. In the fuel cell stack, supplied hydrogen electrochemically reacts with oxygen to generate electrical energy.
In the FP, a hydrocarbon is reformed using a catalyst. If the hydrocarbon includes a sulfur compound, the catalyst is easily poisoned with the sulfur compound. Thus, a sulfur compound must be removed before the hydrocarbon is supplied to the FP. Therefore, a desulfurization process is performed before a reforming process (refer to FIG. 1).
When the hydrocarbon is reformed, not only hydrogen, but carbon dioxide and a small amount of carbon monoxide are generated. However, the reformed fuel should not be directly supplied to the fuel cell stack. Instead, a shift process should be performed to remove carbon monoxide since carbon monoxide poisons catalysts that are used for electrodes of the fuel cell stack. The concentration of carbon monoxide is preferably less than 5000 ppm.
Reactions such as a shift reaction, a methanation reaction and a PROX (preferential oxidation) reaction described in Reaction Schemes 1 through 3 below have been used to remove carbon monoxide (CO).

A temperature of the shift reactor must be 150° C. or higher to lower the carbon monoxide concentration to less than 5000 ppm. However, it takes about one hour to increase the temperature of the shift reactor to the desired temperature. The one-hour wait before using the electrical energy is disadvantageous to the fuel cell system, and thus there is a need to shorten the waiting time. However, a fuel cell system having a sufficiently low carbon monoxide concentration and quick start-up has not been reported.